1. Field of the Invention
This invention relates to polyphase electrical power systems and more particularly to protective relaying apparatus initiating control signals in response to the occurrence of a fault condition on the system.
2. Description of the Prior Art
The actuation of a principal breaker in response to the sensing of a fault by a fault detector normally results in the opening of the principal breaker and the interruption of the fault current which was flowing therethrough. In exceptional circumstances the principal breaker fails to interrupt the fault current and backup breakers must be actuated to prevent loss of stability in, and excessive damage to, the power system. The result of actuating these backup breakers is that the faulted circuit, as well as unfaulted neighboring circuits, is deenergized. The actuation of the backup breakers should therefore be avoided whenever possible. However, if the principal breaker is actually unable to, or for any reason does not, interrupt the fault current the backup breakers must be actuated. When a current greater than a predetermined minimum magnitude flows through the principal breaker at a time interval beyond that established for the breaker to interrupt the fault current, the breaker is presumed to have failed and the backup breakers must be actuated to interrupt the fault current.
A substantial portion of the prior art uses either one of two schemes for producing a control signal for the actuation of the backup breakers. The first scheme initiates a control signal at the end of a fixed and predetermined time period measured from the occurrence of a fault condition. The time period is chosen such that protection is provided in the event of the worst possible expected conditions. This scheme provides for a minimal amount of time between the failure of the primary breaker and the actuation of the backup breakers thus assuring system stability at the expense of unnecessary tripping. The net result is the premature actuation of the backup breakers whenever there is a fault which does not seriously threaten system stabiltiy.
The second scheme provides a control signal at the end of a time period that is inversely proportional to the magnitude of the largest fault current. Accordingly, when there is a large fault current which threatens system stability the time between the failure of the principal breaker and the actuation of the backup breakers is significantly less than when there is a small fault current which does not threaten system stability. Such a scheme is illustrated in U.S. Pat. No. 3,666,994. The overcurrent protective device disclosed therein is responsive to the overcurrent of greatest magnitude in a polyphase electrical circuit.
Overcurrent protective devices which initiate control signals at the end of a time period that is inversely proportional to the magnitude of a fault current provide for the fast actuation of backup breakers for severe high current faults and at the same time provide for slower actuation in less critical situations. However, current magnitude in and of itself is not always a good indicator of the threat to system stability. A better indication is the number of faulted phases in addition to the magnitude of the fault currents. The invention disclosed herein is a current detector that is responsive to both current magnitude and the number of faulted phases.